(1) Field of the Invention
This invention generally relates to processing of hemicellulosic materials for producing purified cellulosic products including purified pulp and pure cellulose for industrial use. Specifically, the invention is concerned with a method of removing lignin from hemicellulosic materials in the processing thereof.
(2) Description of the Prior Art
Conventional methods of processing lignocellulosic products generally include one or more refining steps and the preferred prior art method of obtaining a product of sufficient brightness is bleaching by means of chlorine-based bleaching agents. Further, oxygen compounds, such as peroxidic or peracetic agents and ozone(O.sub.3 ) have been suggested for bleaching of cellulosic materials. Use of chlorine-based bleaching agents has met with increasing objections for ecological reasons and various attempts were made to replace chlorine bleaches in lignocellulose processing. Use of ozone as a supplemental or complemental bleaching in the paper industry has been discussed repeatedly in scientific papers, cf. Abstract Bulletin of the Institute of Paper Chemistry: Vol. 47, No. 6, page 686, abstract 6669; Vol. 53, No. 3, page 331, abstract 2966; Vol. 53, No. 8, page 916, abstract 8417; Vol. 54, No. 6, page 672, abstract 6233; Vol. 55, No. 8, page 937, abstract 8883; and Chemical Abstracts, Vol. 103, No. 6, page 88, summary 38887c.
Ozone for treating of wood is disclosed in U.S. Pat. No. 4,119,486 where the ozone is used for bleaching pulp in the form of an aqueous slurry and in the presence of specific surfactants. U.S. Pat. No. 4,372,812 teaches use of ozone in a chlorine-free bleaching method for lignocellulosic pulp where ozone bleaching stages are followed by aqueous extraction with alkaline (caustic) agents.
Published German patent application No. 3,445,132 discloses a method of preparing pulp by percolating wood at an elevated temperature and ambient pressure with a digestion liquid composed of aqueous acetic acid and a small portion (0.05% to 0.2%) of hydrochloric acid; near the end of the treatment a minor portion (0.5% to 2%) of hydrogen peroxide may be added to the percolating liquid for brightening effects so as to avoid use of chlorine bleaches. This method has since become known and accepted in the art as the "acetosolve" process.
It should be noted that conventional concepts of "bleaching" imply discoloration rather than physical removal of impurities that are present in an otherwise uncolored or "bright" base material. The present invention, on the other hand, is concerned with actual removal of a substance, i.e. lignin, in the course of processing lignocellulosic materials, and the use of ozone as a bleaching agent should be distinguished from its use as a reactant for selective modification of certain properties of the lignin contained as a component in a lignocellulosic material In this context, the term "lignocellulose" is intended to refer to generally macromolecular substances which contain hemicellulosic materials as well as lignin in addition to cellulose.
Further, previous efforts of using ozone as a bleach in commercial pulp production have generally suffered from the fact that ozone in the slurry water present in bleaching steps according to the art reacts with hydroxyl ions to form hydroxyl radicals which, in turn, cause degradation of the cellulose molecules and hence a deterioration of the strength properties of the pulp fibers. Such deterioration effects have, in fact, promoted the belief that ozone is not an advantageous bleaching means for pulp production.
On the other hand, it was found that the pulp obtained by digestion according to the acetosolve process with acetic acid/hydrochloric acid as the digestion medium may contain up to about 4% by weight of residual lignin which preferably is treated with peracetic acid formed upon addition of hydrogen peroxide to the acetic acid in which the pulp is slurried after termination of the first stage of the acetosolve process. Peracetic acid has been found to be a selective oxidizing agent for lignin, i.e. without causing oxidative deterioration of the non-lignin constitutents, notably the cellulose. On the other hand, relatively large amounts of hydrogen peroxide may be needed in the acetosolve process to achieve a satisfactory degree of lignin removal; further, a satisfactory degree of brightness is generally achieved with the hydrogen peroxide addition only when processing hardwood pulp rather than softwood pulp.